sudden change in water temperature to hot

Just in the last couple of days have noticed that hot water is getting extremely hot. have one valve shower head and have to turn it to cold side to get warm water otherwise hot burns. Checked other faucets and hot is really hot; much hotter than normal. Two of the other faucets are one valve/handle and the other is two handle. Doesn't appear to be any leaks that we can find easily. Thermostat in hot water heater? Need help!

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Assuming no adjustments have been made to the temperature setting of the water heater? There could be something keeping the burner on in the water heater. I'm no good at water heater guts but I'm thinking thermocouple.